Enhancing the mobility of liquid droplets on rough surfaces is of great interest in industry. Applications range from condensation heat transfer to water harvesting to the prevention of icing and frosting. The mobility of a liquid droplet on a rough solid surface has long been associated with its wetting state. When liquid drops are sitting on the tips of solid textures and air is trapped underneath, they are in the Cassie state. When the drops are impregnated within the solid textures, they are in the Wenzel state. The Cassie state has been associated with high droplet mobility, while the Wenzel state has been associated with droplet pinning.
Many plants, insects, and animals have highly liquid repellent surfaces, with well-known examples including lotus leaves, the legs of water striders, and the feet of tokay geckos. The liquid repellent function of these natural surfaces is attributed to the presence of hydrophobic hierarchical micro- and nanoscale surface textures that maintain liquid droplets in the Cassie state. Surface textures yielding Cassie state droplets are surfaces with superhydrophobic or superomniphobic properties, with a typical liquid contact angle over 150° and contact angle hysteresis less than 10°. Liquids on these surfaces can roll off with minimal tilting owing to the reduced liquid-solid contact area. Inspired by these natural surfaces, a range of engineered superhydrophobic or superomniphobic surfaces have been developed over the last decade with technological applications ranging from self-cleaning surfaces to drag reduction coatings.
Liquid droplets on rough surfaces typically exhibit Cassie state, Wenzel state, or a combination of these two states. It is often desirable to maintain high liquid repellency in industrial applications, such as fog harvesting, dropwise condensation, and anti-icing. Because the conventional Wenzel state has long been associated with droplet pinning, intense research has focused on maintaining liquid droplets in the Cassie state. Sustaining a droplet in this state is difficult under certain conditions, however, as the air layer underneath the droplets can be disrupted when subjected to high pressure or high temperature, or when encountering liquids with impurities or low surface tensions. Once the air layer is depleted, the liquid will impregnate the solid textures. As a result, the liquid strongly adheres to the solid surface due to the increased contact area of the liquid-solid interfaces and liquid pinning at defects in the solid substrate.
Once a droplet is in the conventional Wenzel state on a roughened surface, it becomes immobile. In an effort to recover droplet mobility, previous researches have been predominantly focused on the ways to induce Wenzel-to-Cassie transition. Thus far, very few design strategies can restore the liquid from the fully impregnated Wenzel state to the Cassie state, and strategies successful in doing so require the use of external energy.
Several publications disclose slippery liquid-infused porous surfaces (SLIPS) to repel liquids. See, e.g., WO2012100099, WO2012100100, WO2013115868 to Aizenberg et al. Other publications disclose methods and compositions related to liquid repellant surfaces having selective wetting and transport properties. See, e.g., WO2014012078, WO2014012079 to Aizenberg et al. Additional references disclose liquid-impregnated surfaces with non-wetting properties. See, e.g., WO2013022467, WO2014145414 to Smith et al. and Kim et al., Hierarchical or Not? Effect of the length Scale and Hierarchy of the Surface Roughness on Omnbiphobicity of Lubricant-infused Substrates, Nano Letters, 2013:13:1793-99.
However, there is a continuing need for technology that provides a simple solution to maintain droplet mobility without requiring challenging transitions, and for new liquid-repellent surface designs.